It is well known in the field of optics that optical elements suitable for the purpose of guiding and focusing light can be manufactured from amorphous materials, such as glass or plastic, by imparting curvature to the optical faces of the elements to form lenses. To impart a precise curvature which is required for most optical applications, various grinding, molding, and polishing techniques must be used. These techniques are often costly and difficult to perform.
It is also known that imparting a gradient in the refractive index (RIG) of an optical element in a direction perpendicular to the direction in which electromagnetic radiation propagates, i.e., perpendicular to the optical axis of the element, renders it suitable for guiding and focusing light without the necessity of imparting curvature to the optical faces of the element. The optical axis of an element is parallel to the direction in which electromagnetic radiation propagates through the element. Such elements can be used to replace standard curved lenses. RIG techniques can also be combined with conventional lens-making technology, e.g., grinding and molding, to form lenses having curved surfaces, but which possess optical properties that are otherwise difficult to achieve via conventional lens-making processes.
Known processes for manufacturing RIG elements involve mixing or diffusing a foreign material into the element matrix. By creating a concentration gradient of the foreign material in the matrix, a refractive index gradient is imparted to the element. A review of known RIG element manufacturing techniques can be found in D. T. Moore, "Gradient-Index Optics: A Review", Applied Optics, 19, 7, April, 1980. The techniques are divided into six categories: (1) neutron irradiation, (2) chemical vapor deposition, (3) polymerization techniques, (4) ion exchange, (5) ion stuffing, and (6) crystal growing.
The diffusion of a material into a glass element is disclosed in U.S. Pat. No. 3,486,808, in which an optical RIG element is produced by treating the surface of a borate or alkali silica glass with a molten salt containing cations of silver or thallium. As the cations diffuse into the glass, the resulting ion exchange process causes the co-ion (e.g., alkali) in the glass to be replaced by the counter-ion (e.g., silver). The concentration gradient of the counter-ion and the RIG can be made approximately linear. A similar ion-exchange technique, involving the replacement of lithium ions contained in the glass with sodium ions from a fused salt bath, is described by Pearson et al. in "Preparation of a Light Focusing Glass Rod by Ion Exchange Techniques", Applied Physics Letters, 15, 76, July, 1969.
U.S. Pat. No. 4,277,271 discloses a method of manufacturing a RIG optical fiber which involves enveloping a core glass with a cladding glass having a different refractive index. The glasses are melted at a temperature high enough to cause partial mixing of the cladding and core glasses. The mixed glasses have an index of refraction somewhere between that of the core and the cladding glasses. The temperature is then elevated further and the glasses are drawn into a fiber.
The diffusion of a material into a plastic element is also used to produce optical elements with refractive index gradients. U.S. Pat. No. 3,718,383 describes the manufacture of a plastic lens-like optical element with a RIG. In this method, a diluent of a low molecular weight organic fluid, such as ethylene diacetate, is diffused into a transparent polymeric matrix, such as poly(methyl methacrylate). The diluent is distributed by diffusion into the matrix to form a continuous gradient of refractive index.
A process for making a polymeric optical RIG element by diffusing two copolymerizable monomers having different refractive indices, and then polymerizing, is described in U.S. Pat. No. 4,022,855. The first monomer is placed in a mold rotating fast enough to hold the monomer against the outside edge of the mold. The second monomer, having a different refractive index than the first, is injected into the center of the mold, and the rotation is slowed to cause diffusion of the two monomers. Ultraviolet light is used to initiate polymerization and adjust the rate of diffusion by controlling the rate of viscosity increase. Polymerization is then completed by heat curing.
The above techniques often require long times and great expense to establish the proper gradients. The present invention allows an optical RIG element to be prepared without the need to mix or diffuse a second material into the element matrix.